Mediterranean diet adherence and sleep pattern: a systematic review of observational studies

Background and aims Despite a huge body of evidence on the linkage between dietary intakes and pattern of sleeping, the findings are controversial. The current study aimed to summarize earlier findings on the association between adherence to Mediterranean diet (MD) and pattern of sleeping. Methods This study performed based on PRISMA guideline. Systematically search was applied in PubMed, Scopus and Google Scholar to find out relevant publications appeared up to February 2023. No restrictions on language and time of publication were applied. Duplicate citations were removed. We included observational studies which assessed MD as the main exposure and kind of sleep disorders as the main outcome. Results A total of 20 observational studies included. Out of these studies, two were cohort studies and 18 had a cross-sectional design. A total of 21,714 participants included. Usual dietary intakes were assessed using a validated Food Frequency Questionnaire, and a diet history questionnaire. Some studies did not report methods of measuring habitual dietary intakes. Adherence to MD was evaluated by KIDMED questionnaire, PREMED, alternate Mediterranean (aMed) questionnaire, MEDAS questionnaire, MedDietScore, MEDI-LITE score, modified Mediterranean Diet Score (mMDS), Mediterranean food pattern (MFP) and modified Mediterranean diet score (mMED). Pattern of sleeping was examined as sleep quality, sleep duration, sleep latency, sleep efficacy, sleepiness, sleep disturbance, taking a nap and some other sleep disorders. Conclusion In conclusion, findings of published studies highlighted the importance of consumption of MD for better sleep quality.


Intruduction
Sleep is an important physiological function to repair and clear tissue and brain [1].World Health Organization expressed that 27% of the world's population suffer from sleep disorders [2,3].Other sleep disorders include sleep-disordered breathing (SDB), parasomnias, narcolepsy, and restless leg syndrome [4].As a common health problem in modern society, sleep disorder have a high incidence in the elderly population, which seriously affect the quality of life and physical and mental health [5].There is a high prevalence of sleep disturbance worldwide as 7.8% of adults had severe sleep problems in the general population [6].The prevalence of sleep disorder in older Iranian adults was reported as 48.9% [7].Medical conditions, depression, anxiety, or cognitive dysfunction can occur along with sleep disorders [8].These disorders can increase the risk of stroke [9,10], migraine [11], neurodevelopmental disorders [12] and irritable bowel syndrome [13].Despite the role of several factors in sleep quality, including age, sex, body weight and depression [14], dietary factors have received great attention recently.Consumption of fruit, vegetables, dairy products and various vitamins and minerals have been previously studied in this regard [15].However, limited data are available abut theses specific dietary patterns.The Mediterranean diet (MD) is a diet based on high consumption of green leafy vegetables, fruits, fish, healthy fats mainly olive oil, legumes, whole grains, nuts and seeds, moderate intake of dairy products and wine consumption as well as low consumption of processed foods, confectionery and red meat [16,17].This diet is low in saturated fat and high in vegetable oils, which observed in Greece and Southern Italy during the 1960s [2].The MD consists of antioxidants, anti-inflammatory micronutrients and n-3 fatty acids and is characterized by a high intake of monounsaturated fat and fiber [18].
The association between MD and various healthrelated outcomes has been previously studied [19].For instance, some documents have shown that the high adherence to the MD can be associated with a lower incidence of chronic diseases and lower aging impairment [20] and frailty [21].MD also can affect against platelet aggregation [22] and mental disorders including cognitive decline and cancer [23].In fact, weight loss programs based on the MD, can decrease the lean tissue losses [24].MD beneficial effects seem to be exerted in both populations of Mediterranean and non-Mediterranean areas [23].
Adherence to the Mediterranean diet has also been studied in relation to sleep patterns; however, findings were conflicting.For example, in a cohort study on the US women aged 20-76 y, adherence to the Mediterranean diet was associated with better sleep quality, higher sleep efficiency, and lower sleep disorders [25].
In contrast, van Egmond et al. in a study on 970 Swedish older men failed to see any significant association between adherence to the MD and self-reported sleep initiation and sleep maintenance problems [26].Despite having several studies in this field, no systematic review has been conducted on the relationship between MD and sleep disorders.Overall, given the presence of conflicting results on the association between MD and sleep disorders, there is a need for a systematic review summarizing all available findings in this field.Considering above, we performed the current study to systematically review all available studies regard to the relationship between MD and sleep pattern.

Search strategy
This study performed based on PRISMA, protocol for reporting systematic reviews.We performed a comprehensive literature search in the online databases of PubMed, Scopus and Google Scholar up to January 2024.The key words which used for this search were as follows: "Mediterranean Diets" OR "diet" OR "Mediterranean dietary pattern" OR "Feeding Behavior" OR "dietary adherence" OR "dietary score" OR "Mediterranean score OR "MD score" OR "food pattern" OR "dietary habit" OR "Mediterranean dietary score" OR "Mediterranean" OR "dietary pattern" accompanied by "Sleep Disorders" or "insomnia" or "Sleep Wake Disorders" or "Sleep Disorders, Circadian Rhythm" or "Sleep" or "Sleep Deprivation" or "Sleep quality" or "Sleep disturbance" or "Sleep quality index" or "Sleep duration" or "Sleep impairment".All keywords were based on MeSH and non-MeSH terms.All references of selected articles were also reviewed to find relevant missing publications.

Selection
No restrictions on language and time of publications were applied.Duplicate citations were removed.Observational studies assessing the association between adherence to MD and sleep pattern were included in this systematic review.We included studies with the following criteria: (1) observational studies with prospective, casecontrol or cross-sectional design; (2) studies that considered adherence to MD as the main exposure; (3) those that had examined every kind of sleep disorders as the main outcome or as one of the outcomes.We excluded letters, comments, short communications, reviews, meta-analyses, ecological studies, and animal studies.A flow diagram of the study selection is shown in Fig. 1.

Data abstraction
Required data from each eligible study were extracted by two independent investigators, and any disagreements were reconciled by discussion.The following information was extracted: name of the first author, publication year, individuals' characteristics (mean age and sex), study design, sample size, method of assessment of exposure, type of study outcome, and method of outcome assessment.

Results
Totally, 3287 articles were found in our initial search; of them 2380 duplicate articles were excluded.After screening the remaining 907 records, 861 unrelated articles were also removed based on title and abstract assessment.Then, 46 articles remained for further evaluation of the full text.Out of those 46 studies, 21 studies were excluded due to assessing MD in relation to sleep apnea and other five studies were also excluded due to having interventional design.Finally, 20 articles were included in the current systematic review.All these studies had examined the association between MD and sleep disorders.Required information for each study has been given in Table 1.All included studies assessed in terms of demographic information as follow including study design, study location and date, exposure assessment,

The association between MD and sleep disorders
Sleep disorders in relation to the adherence to the Mediterranean diet were investigated in different studies as follow:

Sleep quality
Sleep quality was reported in the most included studies.Mamalaki et al. [33] assessed sleep quality by examining following items: daytime sleepiness, sleep adequacy, sleep disturbance, trouble falling asleep, take naps, feel drowsy or sleepy during the day, trouble staying awake during the day, snoring, snore during sleep, awaken during sleep and have trouble falling asleep and awaken short of breath or with a headache.Ferranti et al. [30] reported the association of a component of sleep quality with MD and did not consider total sleep quality.Eleven studies that evaluated the association of MD with sleep quality reported a greater adherence to the MD in association with a better sleep quality [37,[42][43][44].

Sleep duration
Seven publications assessed MD in relation to sleep duration.Six studies reported a significant association between adherence to MD and sleep duration [27,32,35,36,40,41].While three studies reported no significant association between MD and duration of sleep [28,33,44].Castro-Diehl et al. [29] compared moderate-high aMed score to a low aMed score across different categories of objectively measured sleep duration (< 6 h/night, 6-7 h/night, 7-8 h/night and > 8 h/night).They reached no significant association in total; however, participants with a moderate-high aMed score were more likely to sleep 6-7 h/night than those who had a low aMed score.

Sleep latency
Sleep latency was examined in three studies [25,32,44].All of them had reported adherence to the MD was associated with a lower sleep latency.

Sleep efficacy
Based on three studies that assessed adherence to MD in relation to sleep efficacy, two of them reported no significant association [25,44] and the other found a lower occurrence of low sleep efficacy by a greater adherence to MD [32].

Sleepiness
All three studies [30,33,35] that evaluated adherence to MD and sleepiness by PDSS [35] and self-report questionnaires [30,33] reported an inverse association between sleepiness and MD.

Sleep disturbances
Four studies considered sleep disturbances as the outcome of interest.Zuraitkat et al. [25] and Naja et al. [44] found an inverse association between consumption of MD and sleep disturbances among women, however, no significant association was seen between sleep disturbances and MD in Godos et al. [32] and Mamalaki et al. [33] studies.

Taking a nap
All three studies that evaluated the association between adherence to MD and taking a nap reported no statistically significant associations [30,33,36].

Other sleep disorders
In addition to the disorders mentioned above, some other sleep disturbances were also examined in some studies.Needing medications to sleep and self-rated sleep quality [32] as well as sleep initiating problems and sleep maintenance problems [26] were also assessed in any other investigation.In general, only self-rated sleep quality was positively associated with MD.Other sleep outcomes were not associated with this dietary pattern.Mamalaki et al. [33] found no significant association between adherence to MD and some sleep disorders including: not quiet sleep, awaken short of breath or with a headache, feel drowsy or sleepy during the day, trouble falling asleep, awaken during sleep and have trouble falling asleep, trouble staying awake during the day, snore during sleep, snoring and sleep adequacy.However, they found lower occurrence of trouble falling sleep and higher sleep adequacy among those with the greatest adherence to MD than those with the lowest adherence.In the study of Castro-Diehl et al. [29], the association between threelevels (no change, decreased and increase) of change in aMed score was examined in relation to insomnia.No change in aMed score in almost a ten year period, compared with a decreased aMed score in this period, was associated with improved insomnia.Although a significant association was seen between adherence to MD with total sleep time and weekdays sleep time, no significant association was reported between sleep time on weekend and bed time or wake time on weekday and weekend with MD in study of Ferranti et al. [30].

Study quality
The quality of studies included in the current review was assessed using the Newcastle Ottawa Scale (NOS), designed for nonrandomized studies [45].According to this scale, a maximum of 9 points awarded to each cohort study according to the following parameters: 4 points for selection of participants, 2 points for comparability, and 3 points for the assessment of outcomes.A maximum of 10 points awarded to each cross sectional study include: 5 points for selection of participants, 2 points for comparability, and 3 points for the assessment of outcomes.
A study with score from 7 to 9 has high quality, 4-6, has high risk, and 0-3 has very high risk of bias.Based on NOS scoring, we found that 15 studies had high quality [26,29,30,32,33,35,36], four studies had a high risk of bias [25,27,28,34] and one study [31] had a very high risk of bias.

Discussion
In this study, we summarized earlier studies about the adherence to MD and sleep quality.Summarizing previous findings, we found that adherence to MD might help sleeping better.Sleep disturbance can be linked to poor health outcomes and increase the risk of developing metabolic disease and cardiovascular events [46].Nutrition can profoundly affect the hormones and inflammation which directly or indirectly contribute to good or bad sleep quality [47].MD has long been studied in relation to several health related outcomes including sleep hygiene [47].The MD was first defined as a diet with low saturated fats and high vegetable oils [48] with a particular focus on extra virgin olive oil.It contains high amounts of vegetables including leafy green vegetables, fruits, cereals, nuts and pulses/legumes, moderate intakes of fish and other meat, dairy products and red wine, and low intakes of eggs [47,49].It has been supposed that MD can increase the secretion of brain derived neurotrophic factors and improve total body antioxidant capacity [50].Polyphenols intake in this dietary pattern can help explaining its Anti-inflammatory and antioxidant properties, through which it might affect learning and memorizing [51][52][53].MD contains dietary sources of tryptophan, which is an amino acid that is associated with improving sleep quality [52].Consumption of MD is accompanied with less sleep disorders because of its specific dietary components as high levels of very long-chain n-3 PUFAs [54] that might induce secretion of melatonin and serotonin with their fundamental role in better sleep quality which modulates circadian rhythm [50,55].MD can also improve adiposity and body weight, blood pressure, blood lipids, glucose metabolism and insulin sensitivity that may beneficially affect brain function, cognition, and mood which are also important to sleep [56,57].Moreover, it is considered that gut microbiota mediate sleep effects of the MD as some of them can promote higher production of SCFA and serotonin, and improve oxidative stress, inflammation, neurologic, and cognitive functions [58,59].In addition, olive oil favorably changed gut microbiota composition and metabolic function, maybe by increasing SCFA production [58].
We were not able to do a meta-analysis on these studies because of heterogeneity between studies in terms of reporting different effect sizes.In addition, the study design in included studies were different, which prohibited us again to derive a quantitative assessment of available literature.Most included studies, except for two, did not use actigraphy and Wrist-worn accelerometer to measure sleep quality, which are non-subjective method for assessment of sleep quality.Others had mostly used self-reported questionnaires to examine sleep quality, which are subject to recall and social biases.About assessing dietary pattern all studies had used selfreported questionnaires.Therefore, misclassification of study participants is unavoidable.The generalizability of our findings should be done cautiously because most included studies had examined a particular age group or had limited the analysis to one gender.Our quality assessment of included studies indicated that out of 12 studies, only seven studies had a high quality and the five remaining studies had a high risk or a very high risk of bias, which might further limits the reliability of findings in earlier studies in this regard.
In conclusion, most findings of published studies highlight the importance of consumption of MD for better sleep quality.Given the different forms of sleep pattern examined in the earlier studies, future large-scale, international, multicenter, population-based, epidemiological studies with samples from different areas like as other countries worldwide, urban, rural and island regions, are essential for more reliable conclusions.Clinical intervention studies to examine the effect of consumption of MD on sleep quality are needed.More animal studies can provide a better view of the mechanisms mediating the association between MD and sleep features.In addition, objective neurophysiological tools for sleep assessment (for example actigraphy, polysomnography) are suggested to widely use in feature studies.Moreover, other studies could examine the effects of meal timing and frequency, in the relation of the MD and sleep quality and quantity.Face-to-face interviews with validated questionnaire such as PSQI is recommended in future studies to reduce recall bias and to increase the validity of the responses.By further studies have been done on these desired variables, meta-analysis would be allowed to be performed.Therefore we can obtain more accurate information and make stronger recommendations for dietary pattern and sleep hygiene.

Fig. 1
Fig. 1 Flow chart of participation

Table 1
Characteristics of included studies on the association between consumption of Mediterranean diet and pattern of sleeping

Table 1 (
continued) assessment of adherence to Mediterranean diet and outcome assessment.